RIGHT SPEAK: 2016 Presidential Hopefuls: Hidden Achilles' Heels

"Every candidate may be wonderful,'' Mitt Romney said recently of the 2016 presidential field in a Fox News interview. "But they all have a but'' — a pun the 2012 GOP nominee did not need to spell out.

TRUMP 1.0

The candidates tell us plenty about their strengths. They're more reticent about their weaknesses (their "buts''), including the less obvious ones.

Every candidate has such an Achilles' heel, a secondary or tertiary liability that can come suddenly to light in a presidential campaign, particularly after the early primary winnowing. It can be a little-known misdeed, personality trait or biographical detail.

Example: In 1999, John McCain was far behind in the GOP presidential polls, riding the Straight Talk Express campaign bus, and charming reporters with his candor and accessibility. Few in his campaign worried about the Arizona senator's formidable temper.

But after his candidacy caught fire, McCain's "emotional balance'' suddenly and unexpectedly became an issue, recalls Dan Schnur, who was campaign communications director. "When we were at 5% (in the polls),'' he says, "we didn't worry about it.''

"Smart campaigns understand their candidates' weaknesses at an intellectual level,'' says Schnur, now director of USC's Unruh Institute of Politics. "But it's hard to process them viscerally. You believe in your candidate, and you're surrounded by others who believe.''

Such weaknesses are often hidden in plain sight or obscured by more glaring ones — such as Bridgegate (Chris Christie) or "Oops!" (Rick Perry). Apparent to insiders and cognoscenti, they can be unseen by the average voter, because the election is so far away and the candidates (currently five Democrats, 15 Republicans) are so many.

And sometimes a weakness might be a strength. Is Scott Walker's lack of a college degree a liability or a badge of honor? If Ted Cruz is unpopular with his congressional colleagues, who as a group are unpopular with the public, is that a plus?